High-pressure sodium (HPS) lamps have a high efficiency and broader spectrum of light than many other types. HPS lamps use combined xenon, mercury and sodium under high pressure to provide a more complete spectrum of light than low-pressure sodium lamps.
HPS lamps are not designed to be interchangeable in other fixtures. HPS bulbs require a specific ballast for startup and proper voltage while burning. HPS bulbs can be rated for 1,000 watts or more. Their high light output per voltage (lumens per watt) makes them a common choice for street lights and indoor growing lights.
On startup, HPS bulbs tend to be pinkish, warming up to a soft yellow after a few minutes. HPS bulbs provide more light in the blue end of the spectrum than low-pressure sodium bulbs. The color or temperature of the light is rated in Kelvins. Higher Kelvin temperature is heavier in blue light. Daylight is considered to be about 5,000 degrees Kelvin. HPS bulbs have a color temperature between about 2,000 K and 3,500 K.
Sodium naturally emits light from the "D-band," which is centered at wavelength 589 nm in the yellow range. Light is emitted as electrons transition from the 3p to the 3s orbitals in elemental sodium. Under high pressure, this band is broadened to include more colors, giving HPS lamps a less-yellow cast and a higher temperature color.
Mercury in HPS lamps provides more violets, blues and greens in the light. Mercury has conspicuous emission at about 405, 435 and 545 nanometers. These are important wavelengths for photosynthesis and are one reason why HPS lamps are used as grow lights.
Xenon has numerous spectral emission lines throughout most wavelengths of visible light. These additional emissions give HPS lamps a more balanced, whiter glow than low-pressure sodium lamps. Xenon is also used in high-intensity discharge lamps and produces the highest color temperature of all the noble gases.
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